OBJECTIVE: To develop methodologies for the isolation of natural killer (NK) cells from the monkey uterus. RESULTS The mechanisms which promote the development of maternal-fetal immune tolerance in primates is not well-understood, but it has been hypothesized that one important component is the inhibition of uterine NK cells by trophblast MHC class I molecules. Previous studies demonstrated that the rhesus monkey placenta expresses a unique MHC class I molecule which has been designated Mamu-AG. In order to investigate the biological role of this molecule in primate pregnancy, we sought to establish methods for the isolation of rhesus monkey endometrium NK cells. Trypsin-DNase digestion of minced endometrial/decidual tissue proved to very efficiently disperse decidual cells, and gradient centrifugation yielded a population which was enriched (78%) in the NK marker CD56, and had low (10%) T/B cell contamination. Following in vitro enrichment, density gradient centrifugation yielded highly pure (>95%) CD56+ cells with less then 1% CD3+ lymphocytes. Uterine NK cells were effective at killing standard NK cell targets, including K562 and Raji cells. These results will be useful in developing approaches to examine rhesus trophoblast-uterine interactions in the establishment of maternal-fetal immune tolerance. FUTURE DIRECTIONS We will develop clones of uterine NK cells to define interactions between NK cell receptors and trophoblast MHC class I molecules. We will clone these receptors from NK cells and define their phylogenetic relationship with human and rodent NK cell receptors. KEY WORDS natural killer cell, maternal-fetal immune tolerance, endometrium, placenta, major histocompatibility complex